1. Field of the Invention
The present invention relates to an ink-jet head which is constructed to distribute an ink supplied to a common ink chamber, and then discharge the distributed ink from nozzles, and an ink-jet printer which uses this ink-jet head.
2. Description of the Related Art
As an ink-jet head according to a prior art, there has been known a structure such as a piezoelectric ink-jet head described in Japanese Patent No 3196800 (corresponding to U.S. Pat. No. 5,748,214), in which an elastic plate is mounted on a cavity unit which includes a plurality of nozzles arranged in rows and pressure chambers for the nozzles, respectively, so that the elastic plate covers all the pressure chambers, and in which a piezoelectric element is provided on the elastic plate to correspond to each of the pressure chambers.
In the cavity unit, the ink supplied from an ink supply source is stored once in a common ink chamber which is provided in the cavity unit lengthwise along a direction of the nozzle rows. Next, the ink is distributed to each of the pressure chambers provided on a rear surface side of the cavity unit, and then the ink reaches each of the nozzles provided on a front surface side of the cavity unit. Further, for realizing such an ink channel in the cavity unit which is a very small component, the cavity unit is formed by laminating (stacking) a plurality of plates, each of which is provided with through holes and recesses of various sizes, with flat surfaces of the plates facing mutually.
The cavity unit in Japanese Patent No. 3196800 is formed such that an ink supply port is open towards the rear side of the cavity unit, as an inlet for supplying the ink from the ink supply source to the cavity unit, and this ink supply port is connected to one end in a longitudinal direction of the common ink chamber. Therefore, the ink which has flowed into the common ink chamber is distributed to the pressure chambers via holes formed in parallel in a direction in which the plates are laminated or stacked (laminating direction, stacking direction), while flowing from the one end in the longitudinal direction of the common ink chamber to the other end in the longitudinal direction of the common ink chamber, along a direction of plane of the plate. In other words, since the ink flows to the pressure chambers upon changing, its direction in which the ink flows in the common ink chamber, to another direction to flow into the pressure chambers by right angles, the ink is stagnated in the common ink chamber at the other end thereof which is the most downstream side in the ink flow, and consequently air bubbles mixed in the ink come or gather together and are easily stagnated. Therefore, there is a problem that, at a time of maintenance, even when a purge operation is performed to forcefully discharge the bubbles in the ink, together with the ink, the air bubbles stagnated at the other end in the common ink chamber are hardly removed.
Therefore, in Japanese Patent No. 3196800, by devising the arrangement of the holes (channel constricting holes in Japanese Patent No. 3196800), so that a flow of the ink which makes the air bubbles to be easily discharged, is developed in the common ink chamber. Specifically, among the holes, holes which communicate with nozzles, respectively, for image recording are arranged in a row along one wall surface in a longitudinal direction of the common ink chamber; and among the holes, a plurality of holes (three holes in FIG. 3) which communicate with a nozzle for air discharge (described as “air discharge port”) are arranged, along a direction of a width of the common ink chamber, at an end of thereof located at the most downstream side in the longitudinal direction of the common ink chamber. The air discharge ports are arranged along a direction orthogonal to the nozzle row at an end of the nozzle row for image recording, corresponding to the arrangement of the holes for the air discharge. In this structure, when the purge operation for forcibly discharging the ink from the air discharge ports and the nozzles is performed, a flow of the ink (ink flow) is generated in the common ink chamber at the most downstream side end thereof, throughout the width of the common ink chamber, and the air bubbles, without being stagnated, are discharged promptly from the air discharge ports via the holes for the air discharge.
As in the ink-jet head described in Japanese Patent No. 3196800, in a structure including air discharge ports in a surface at which the nozzles are open, it is desired to improve the efficiency in discharging the air bubbles from the air discharge ports. For improving dischargeability of air bubbles in the air discharge ports, it is necessary to reduce a channel resistance in the air discharge port to be lower than a channel resistance in the nozzle for the image recording. For this purpose, it is conceivable, for example, to increase the number of air discharge ports, to increase an area of opening of the discharge ports to be greater than an area of opening of the nozzle for the image recording, and the like.
However, in ink-jet heads developed in recent years, there is a tendency that a planar area of the ink-jet head as a whole is reduced to be small in response to the reduction in size of the entire ink-jet head and the high densification of recording, whereas a space between the nozzle rows for image recording tends to be narrow due to the increase in the number of nozzles and nozzle rows. In the ink-jet head described in Japanese Patent No. 3196800, the plurality of air discharge holes and the plurality of air discharge ports are arranged along the width direction of the common ink chamber. In this arrangement, however, when the space between the rows becomes narrow, there is a limit up to which the space can be narrowed, and it has been difficult to increase the number of the holes for the air discharge and the air discharge ports.
On the other hand, when the air discharge ports are made to have an opening having a wider area than that of the nozzles for image recording, a meniscus pressure resistance is lowered only in the air discharge ports. In this case, a back pressure (negative pressure) acts to the ink in the ink-jet head as it has been publicly known, and further when there is a pressure fluctuation or the like, the meniscus is destroyed easily and outside air enters the common ink chamber through the air discharge ports, thereby forming a lump of air bubbles in the common liquid chamber. Then, the lump of air bubbles formed in the common ink chamber blocks the holes (channel constricting holes) each communicating with one of the nozzles for image recording, thereby causing a problem of defective ink jetting or discharge.